1. Field of the Invention
The present invention relates to a focus servo pull-in apparatus which serves to start a focus servo operation from an optimum position according to the disk type.
2. Description of the Related Art
The developments of DVD (Digital Video Disk or Digital Versatile Disk), which is a high-density recording medium having a storage density significantly higher than CDs (Compact Disks), are actively performed so that a single DVD is able to record the entirety of a single movie or the like. In a DVD, the distance from the surface of the optical disk to the information recording surface is about 0.6 mm, while in a CD the distance is 1.2 mm. Because both DVDs and CDs are designed on the same principle of reading recorded pits that bear information, there have been competing propositions on CD/DVD compatible players capable of playing back both DVDs and CDs. The proposed CD/DVD compatible players include a type which changes two kinds of lenses with different focal distances in accordance with the type of the optical disk to be reproduced, and a type which uses a bifocal lens comprised of a holographic optical element, in order to optimally converge the information reading beam on the information recording surface on each optical disk.
For example, a bifocal lens using a holographic optical element (diffraction grating) is designed with a diffraction grating 13 and an objective lens 11 arranged on the same optical path as shown in FIG. 1A. A light beam converted to parallel rays by a collimator lens 14 is split by the diffraction grating 13 into three beams, namely a zero order beam, +first order beam and -first order beam (-first order beam not shown) of which the zero order beam and +first order beam converge at different focal points on a same line using the difference between the lengths of their optical paths.
Specifically, as a +first order beam converges at a position farther from the objective lens 11 than a zero order beam with respect to the information recording surface of a DVD or CD, the bifocal lens is set in such a way that the zero order beam optimally focuses on the information recording surface of a DVD while a +first order beam optimally focuses on the information recording surface of a CD.
Let us consider the case where an optical pickup using this bifocal lens is gradually moved away from an optical disk. As shown in FIG. 1B, first, the zero order beam is irradiated on the information recording surface of the optical disk, an unillustrated quadrant photodiode provided in the optical pickup of a disk player sends out an S-shaped signal or a focus error signal. When the optical pickup is moved away further, the reflected light of the +first order beam from the optical disk returns through the optical path of the zero order beam, yielding a pseudo S-shaped signal. Finally the S-shaped signal corresponding to the +first order beam can be obtained.
FIGS. 2A through 2C illustrate how the S-shaped signal of a zero order beam, the pseudo S-shaped signal and the S-shaped signal of a +first order beam mentioned above appear when the bifocal lens is moved away from the optical disk at a specified speed. The dividing ratio of the zero order beam to the +first order beam is 1:1.
In the case of a bifocal lens, the optimal condensation of the zero order beam is done to a DVD, and the optimal condensation of the +first order beam is done to a CD. Provided that the level of a focus S-shaped signal yielded from the zero order beam is 100% when the zero order beam is irradiated on the information recording surface of a DVD, this level does not become 100% but smaller when the zero order beam is irradiated on the information recording surface of a CD, as shown in FIGS. 2B and 2C.
Likewise, given that the level of a focus S-shaped signal yielded from the +first order beam is 100% when the +first order beam is irradiated on the information recording surface of a CD, this level becomes smaller than 100% when the +first order beam is irradiated on the information recording surface of a DVD. In the case of a bifocal lens, as apparent from the above, the amplitude of the focus error signal detected by the optical pickup varies and the ratio of the zero order beam to the +first order beam, (zero order beam/+first order beam), for a DVD becomes greater than the ratio of the zero order beam to the +first order beam, (zero order beam/+first order beam), for a CD.
The distance, D-WD, from the surface of the objective lens to the disk surface when the light beam is focused on the information recording surface or a working distance (WD) for a DVD may differ from the working distance C-WD for a CD.
If focus lock control of the pickup is performed on a DVD and a CD with the same focus drive voltage, focus lock point on the DVD differs from that on the CD. This undesirably delays the focus lock time tC for the CD as compared with the focus lock time tD for the DVD.
While this shortcoming may be overcome by matching D-WD with C-WD, pickups are generally designed in particular consideration of suppressing the occurrence of aberration in order to improve the reading of information signals from disks and such a design conflicts with matching D-WD with C-WD. In this respect, the mentioned problem still remains.